apatite

The chief phosphate mineral
(calcium phosphate associated with fluoride, chloride, or hydroxyl ions),
large deposits of which are found in the Kola peninsula, Russia, North Africa,
and Montana and Florida.Apatite is found in igneous
rocks and sedimentary deposits. It is
mined for fertilizer and is the major
ore of phosphorus.

The chemical composition of
apatite is Ca5(PO4)3X, where X = F (fluorapatite,
the most common), Cl (chlorapatite), OH (hydroxyapatite), or a mixture of
all three; it forms prismatic or tabular hexagonal crystals, and also occurs
as granular aggregates or in massive crusts. It is usually too soft for
cutting and polishing, though there are two gem varieties. Hardness 5, specific
gravity 3.1–3.4.